If I were to connect an ignition coil directly to a battery

...

Each time you break the connection to the battery, the plug will fire.

I sometimes bench-test coils by connecting the coil to a battery and sparkplug.
Or better, to a spark-tester with a much wider gap of 10mm or more.

This is a good way to install a flame thrower. Just need a source of gas or propane, a metal tube and you are in business

Cars would sometimes get a spark plug installed in the side of the tail pipe, downstream
of a propane bottle connection. Don't tailgate.

For the flame thrower they used a model T coil that had points that vibrated all the time to drive the flame thrower plug.

Or you could use a normal 12 V relay with 2 sets of contacts, one set wired as a buzzer, the other drives the coil.
If you used it a lot, it would not last long but for a novelty it is perfect When gas was 20 cents a gallon this was fun.

Rod

a proper flame thrower, is run by gasoline, or petrol, force fed by compressed air into the petrol tank, to push it out. pump and ignition were two separate buttons too, so u could spray petrol 100 meters away, yes they do travel a bloody long way, not like movies, to run into a bunker, and then ignight it.

ive often thought about a efi fuel pump 12v battery, to make something, quight illegal...

Since you don't mention if you want to make things go boom as above, or jury rig an existing ignition system, if you hook it up and leave it powered,
1. The engine won't turn off when you turn the key off.
2. You'll burn the coil up if its "on" and not running.
3. Drain your battery if the engine isn't running.

One more little goodie. 6V car coils are meant to run at ~6.7 volts. 12V car coils are built to run at ~9-10 volts continuously. They'll take 12-13 volts intermittently.

Off topic, but my understanding is that flame throwers are actually legal (used in agriculture or something or other). As a topic, it comes up in the anti/pro-gun rants. Sure, why ban X when flame-throwers are legal?

On topic again... a coil is just a transformer. They only work when the input is changing. 12v, just connected - not changing, will just heat the coil up, nothing at the output at all. You need to make and break the input to get spark on the output.

David...

ah different countries

That's why every owner of an old Chrysler product always carried a spare ballast resistor in the glove box or, a jumper wire to get to the parts store.

+1. I was wondering how long it would take before someone jumped in on this. Let's wait and see if anyone can explain why they did it that way.

when the starter engages, the battery voltage drops, so the coil is designed to run on the lower voltage (about 10V) to produce a hot spark. once the engine starts and the system resumes normal voltage, current to the coil is reduced by the ballast resistor. in start mode the coil is wired to the main buss through the key switch. in normal run mode the key switch puts the resistor is in series with the coil so the voltage will be reduced to the coil only

Nailed it.

So why are 6V coils meant to run at 6.7V?

From my experience, to make them difficult to start.

Because that's ~ the normal 6V car voltage when the generator/voltage regulator is in "charge" of things.

Ford put the ballast resistor in the wiring harness, and got the 12 volt starter boost at the starter relay.

Yup and GM had the resistor in the starter solenoid. Terminal "R" to be exact.

Geez there are a bunch of err, "mature" gentlemen on this forum!

...Because of course you'd diagnose the ballast resistor by the symptom that the car would start and it would immediately die when you released the key. IIRC it would be a $6-$9 repair including the part.